big: implemented overlap-tolerant shifts in assembly

- no need to make copies in cases of aliases
- removed deprecated internal shift functions
- minor unrelated simplifications

This change improves pidigits -s -n10000 by almost 20%:

user 0m6.156s (old)
user 0m4.999s (new)

(pidigits -s -n20000 goes from ~25s to ~19s)

R=rsc
CC=golang-dev
https://golang.org/cl/1149041

src/pkg/big/arith.go

@@ -325,10 +325,16 @@ func subVW_g(z, x *Word, y Word, n int) (c Word) {
 
 func shlVW(z, x *Word, s Word, n int) (c Word)
 func shlVW_g(z, x *Word, s Word, n int) (c Word) {
+	if n > 0 {
 		ŝ := _W - s
-	for i := 0; i < n; i++ {
-		w := *x.at(i)
-		c, *z.at(i) = w>>ŝ, w<<s|c
+		w1 := *x.at(n - 1)
+		c = w1 >> ŝ
+		for i := n - 1; i > 0; i-- {
+			w := w1
+			w1 = *x.at(i - 1)
+			*z.at(i) = w<<s | w1>>ŝ
+		}
+		*z.at(0) = w1 << s
 	}
 	return
 }
@@ -336,10 +342,16 @@ func shlVW_g(z, x *Word, s Word, n int) (c Word) {
 
 func shrVW(z, x *Word, s Word, n int) (c Word)
 func shrVW_g(z, x *Word, s Word, n int) (c Word) {
+	if n > 0 {
 		ŝ := _W - s
-	for i := n - 1; i >= 0; i-- {
-		w := *x.at(i)
-		c, *z.at(i) = w<<ŝ, w>>s|c
+		w1 := *x.at(0)
+		c = w1 << ŝ
+		for i := 0; i < n-1; i++ {
+			w := w1
+			w1 = *x.at(i + 1)
+			*z.at(i) = w>>s | w1<<ŝ
+		}
+		*z.at(n - 1) = w1 >> s
 	}
 	return
 }

src/pkg/big/arith_386.s

@@ -101,53 +101,73 @@ E4:	CMPL BX, BP		// i < n
 
 // func shlVW(z, x *Word, s Word, n int) (c Word)
 TEXT ·shlVW(SB),7,$0
+	MOVL n+12(FP), BX	// i = n
+	SUBL $1, BX		// i--
+	JL X8b			// i < 0	(n <= 0)
+
+	// n > 0
 	MOVL z+0(FP), DI
 	MOVL x+4(FP), SI
 	MOVL s+8(FP), CX
-	MOVL n+12(FP), BX
-	LEAL (DI)(BX*4), DI
-	LEAL (SI)(BX*4), SI
-	NEGL BX			// i = -n
-	MOVL $0, AX		// c = 0
-	JMP E8
+	MOVL (SI)(BX*4), AX	// w1 = x[n-1]
+	MOVL $0, DX
+	SHLL CX, DX:AX		// w1>>ŝ
+	MOVL DX, c+16(FP)
 
-L8:	MOVL (SI)(BX*4), DX
-	MOVL DX, BP
-	SHLL CX, DX:AX
-	MOVL DX, (DI)(BX*4)
-	MOVL BP, AX
-	ADDL $1, BX		// i++
+	CMPL BX, $0
+	JLE X8a			// i <= 0
 
-E8:	CMPL BX, $0		// i < 0
-	JL L8
+	// i > 0
+L8:	MOVL AX, DX		// w = w1
+	MOVL -4(SI)(BX*4), AX	// w1 = x[i-1]
+	SHLL CX, DX:AX		// w<<s | w1>>ŝ
+	MOVL DX, (DI)(BX*4)	// z[i] = w<<s | w1>>ŝ
+	SUBL $1, BX		// i--
+	JG L8			// i > 0
 
-	MOVL $0, DX
-	SHLL CX, DX:AX
-	MOVL DX, c+16(FP)
+	// i <= 0
+X8a:	SHLL CX, AX		// w1<<s
+	MOVL AX, (DI)		// z[0] = w1<<s
+	RET
+
+X8b:	MOVL $0, c+16(FP)
 	RET
 
 
 // func shrVW(z, x *Word, s Word, n int) (c Word)
 TEXT ·shrVW(SB),7,$0
+	MOVL n+24(FP), BP
+	SUBL $1, BP		// n--
+	JL X9b			// n < 0	(n <= 0)
+
+	// n > 0
 	MOVL z+0(FP), DI
 	MOVL x+4(FP), SI
 	MOVL s+8(FP), CX
-	MOVL n+12(FP), BX	// i = n
-	MOVL $0, AX		// c = 0
+	MOVL (SI), AX		// w1 = x[0]
+	MOVL $0, DX
+	SHRL CX, DX:AX		// w1<<ŝ
+	MOVL DX, c+16(FP)
+
+	MOVL $0, BX		// i = 0
 	JMP E9
 
-L9:	MOVL (SI)(BX*4), DX
-	MOVL DX, BP
-	SHRL CX, DX:AX
-	MOVL DX, (DI)(BX*4)
-	MOVL BP, AX
+	// i < n-1
+L9:	MOVL AX, DX		// w = w1
+	MOVL 4(SI)(BX*4), AX	// w1 = x[i+1]
+	SHRL CX, DX:AX		// w>>s | w1<<ŝ
+	MOVL DX, (DI)(BX*4)	// z[i] = w>>s | w1<<ŝ
+	ADDL $1, BX		// i++
 	
-E9:	SUBL $1, BX		// i--
-	JGE L9
+E9:	CMPL BX, BP
+	JL L9			// i < n-1
 
-	MOVL $0, DX
-	SHRL CX, DX:AX
-	MOVL DX, c+16(FP)
+	// i >= n-1
+X9a:	SHRL CX, AX		// w1>>s
+	MOVL AX, (DI)(BP*4)	// z[n-1] = w1>>s
+	RET
+
+X9b:	MOVL $0, c+16(FP)
 	RET
 
 

src/pkg/big/arith_amd64.s

@@ -103,47 +103,93 @@ E4:	CMPQ BX, R11         // i < n
 
 // func shlVW(z, x *Word, s Word, n int) (c Word)
 TEXT ·shlVW(SB),7,$0
+	MOVL n+24(FP), BX	// i = n
+	SUBL $1, BX		// i--
+	JL X8b			// i < 0	(n <= 0)
+
+	// n > 0
 	MOVQ z+0(FP), R10
 	MOVQ x+8(FP), R8
 	MOVQ s+16(FP), CX
+	MOVQ (R8)(BX*8), AX	// w1 = x[n-1]
+	MOVQ $0, DX
+	SHLQ CX, DX:AX		// w1>>ŝ
+	MOVQ DX, c+32(FP)
+
+	CMPL BX, $0
+	JLE X8a			// i <= 0
+
+	// i > 0
+L8:	MOVQ AX, DX		// w = w1
+	MOVQ -8(R8)(BX*8), AX	// w1 = x[i-1]
+	SHLQ CX, DX:AX		// w<<s | w1>>ŝ
+	MOVQ DX, (R10)(BX*8)	// z[i] = w<<s | w1>>ŝ
+	SUBL $1, BX		// i--
+	JG L8			// i > 0
+
+	// i <= 0
+X8a:	SHLQ CX, AX		// w1<<s
+	MOVQ AX, (R10)		// z[0] = w1<<s
+	RET
+
+X8b:	MOVQ $0, c+32(FP)
+	RET
+
+
+// func shrVW(z, x *Word, s Word, n int) (c Word)
+TEXT ·shrVW(SB),7,$0
 	MOVL n+24(FP), R11
-	MOVQ $0, AX         // c = 0
+	SUBL $1, R11		// n--
+	JL X9b			// n < 0	(n <= 0)
+
+	// n > 0
+	MOVQ z+0(FP), R10
+	MOVQ x+8(FP), R8
+	MOVQ s+16(FP), CX
+	MOVQ (R8), AX		// w1 = x[0]
+	MOVQ $0, DX
+	SHRQ CX, DX:AX		// w1<<ŝ
+	MOVQ DX, c+32(FP)
+
 	MOVQ $0, BX		// i = 0
-	JMP E8
+	JMP E9
 
-L8:	MOVQ (R8)(BX*8), DX
-	MOVQ DX, R12
-	SHLQ CX, DX:AX
-	MOVQ DX, (R10)(BX*8)
-	MOVQ R12, AX
+	// i < n-1
+L9:	MOVQ AX, DX		// w = w1
+	MOVQ 8(R8)(BX*8), AX	// w1 = x[i+1]
+	SHRQ CX, DX:AX		// w>>s | w1<<ŝ
+	MOVQ DX, (R10)(BX*8)	// z[i] = w>>s | w1<<ŝ
 	ADDL $1, BX		// i++
 	
-E8:	CMPQ BX, R11         // i < n
-	JL L8
+E9:	CMPQ BX, R11
+	JL L9			// i < n-1
 
-	MOVQ $0, DX
-	SHLQ CX, DX:AX
-	MOVQ DX, c+32(FP)
+	// i >= n-1
+X9a:	SHRQ CX, AX		// w1>>s
+	MOVQ AX, (R10)(R11*8)	// z[n-1] = w1>>s
+	RET
+
+X9b:	MOVQ $0, c+32(FP)
 	RET
 
 
 // func shrVW(z, x *Word, s Word, n int) (c Word)
-TEXT ·shrVW(SB),7,$0
+TEXT ·shrVW_(SB),7,$0
 	MOVQ z+0(FP), R10
 	MOVQ x+8(FP), R8
 	MOVQ s+16(FP), CX
 	MOVL n+24(FP), BX	// i = n
 	MOVQ $0, AX		// c = 0
-	JMP E9
+	JMP E9_
 
-L9:	MOVQ (R8)(BX*8), DX
+L9_:	MOVQ (R8)(BX*8), DX
 	MOVQ DX, R12
 	SHRQ CX, DX:AX
 	MOVQ DX, (R10)(BX*8)
 	MOVQ R12, AX
 
-E9:	SUBL $1, BX         // i--
-	JGE L9
+E9_:	SUBL $1, BX		// i--
+	JGE L9_
 
 	MOVQ $0, DX
 	SHRQ CX, DX:AX

src/pkg/big/nat.go

@@ -60,6 +60,11 @@ func (z nat) norm() nat {
 }
 
 
+// TODO(gri) Consider changing "make" such that is does not reserve space
+//           for a potential carry; instead callers must provide the correct
+//           m (+1). Should lead to clearer code and shorter allocations on
+//           average.
+
 func (z nat) make(m int) nat {
 	if cap(z) > m {
 		return z[0:m] // reuse z - has at least one extra word for a carry, if any
@@ -219,11 +224,7 @@ func (z nat) mulAddWW(x nat, y, r Word) nat {
 // basicMul multiplies x and y and leaves the result in z.
 // The (non-normalized) result is placed in z[0 : len(x) + len(y)].
 func basicMul(z, x, y nat) {
-	// initialize z
-	for i := range z[0 : len(x)+len(y)] {
-		z[i] = 0
-	}
-	// multiply
+	z[0 : len(x)+len(y)].clear() // initialize z
 	for i, d := range y {
 		if d != 0 {
 			z[len(x)+i] = addMulVVW(&z[i], &x[0], d, len(x))
@@ -534,28 +535,26 @@ func (z nat) div(z2, u, v nat) (q, r nat) {
 
 
 // q = (uIn-r)/v, with 0 <= r < y
+// Uses z as storage for q, and u as storage for r if possible.
 // See Knuth, Volume 2, section 4.3.1, Algorithm D.
 // Preconditions:
 //    len(v) >= 2
 //    len(uIn) >= len(v)
-func (z nat) divLarge(z2, uIn, v nat) (q, r nat) {
+func (z nat) divLarge(u, uIn, v nat) (q, r nat) {
 	n := len(v)
-	m := len(uIn) - len(v)
+	m := len(uIn) - n
 
-	var u nat
-	if z2 == nil || &z2[0] == &uIn[0] {
-		u = u.make(len(uIn) + 1).clear() // uIn is an alias for z2
-	} else {
-		u = z2.make(len(uIn) + 1).clear()
-	}
-	qhatv := make(nat, len(v)+1)
 	q = z.make(m + 1)
+	qhatv := make(nat, n+1)
+	if alias(u, uIn) {
+		u = nil // u is an alias for uIn - cannot reuse
+	}
+	u = u.make(len(uIn) + 1).clear()
 
 	// D1.
-	shift := leadingZeros(v[n-1])
-	v.shiftLeftDeprecated(v, shift)
-	u.shiftLeftDeprecated(uIn, shift)
-	u[len(uIn)] = uIn[len(uIn)-1] >> (_W - shift)
+	shift := Word(leadingZeros(v[n-1]))
+	shlVW(&v[0], &v[0], shift, n)
+	u[len(uIn)] = shlVW(&u[0], &uIn[0], shift, len(uIn))
 
 	// D2.
 	for j := m; j >= 0; j-- {
@@ -583,12 +582,12 @@ func (z nat) divLarge(z2, uIn, v nat) (q, r nat) {
 		}
 
 		// D4.
-		qhatv[len(v)] = mulAddVWW(&qhatv[0], &v[0], qhat, 0, len(v))
+		qhatv[n] = mulAddVWW(&qhatv[0], &v[0], qhat, 0, n)
 
 		c := subVV(&u[j], &u[j], &qhatv[0], len(qhatv))
 		if c != 0 {
-			c := addVV(&u[j], &u[j], &v[0], len(v))
-			u[j+len(v)] += c
+			c := addVV(&u[j], &u[j], &v[0], n)
+			u[j+n] += c
 			qhat--
 		}
 
@@ -596,8 +595,8 @@ func (z nat) divLarge(z2, uIn, v nat) (q, r nat) {
 	}
 
 	q = q.norm()
-	u.shiftRightDeprecated(u, shift)
-	v.shiftRightDeprecated(v, shift)
+	shrVW(&u[0], &u[0], shift, len(u))
+	shrVW(&v[0], &v[0], shift, n)
 	r = u.norm()
 
 	return q, r
@@ -755,15 +754,10 @@ func (z nat) shl(x nat, s uint) nat {
 	}
 	// m > 0
 
-	// determine if z can be reused
-	// TODO(gri) change shlVW so we don't need this
-	if alias(z, x) {
-		z = nil // z is an alias for x - cannot reuse
-	}
-
 	n := m + int(s/_W)
 	z = z.make(n + 1)
 	z[n] = shlVW(&z[n-m], &x[0], Word(s%_W), m)
+	z[0 : n-m].clear()
 
 	return z.norm()
 }
@@ -778,12 +772,6 @@ func (z nat) shr(x nat, s uint) nat {
 	}
 	// n > 0
 
-	// determine if z can be reused
-	// TODO(gri) change shrVW so we don't need this
-	if alias(z, x) {
-		z = nil // z is an alias for x - cannot reuse
-	}
-
 	z = z.make(n)
 	shrVW(&z[0], &x[m-n], Word(s%_W), n)
 
@@ -791,48 +779,6 @@ func (z nat) shr(x nat, s uint) nat {
 }
 
 
-// TODO(gri) Remove these shift functions once shlVW and shrVW can be
-//           used directly in divLarge and powersOfTwoDecompose
-//
-// To avoid losing the top n bits, z should be sized so that
-// len(z) == len(x) + 1.
-func (z nat) shiftLeftDeprecated(x nat, n uint) nat {
-	if len(x) == 0 {
-		return x
-	}
-
-	ñ := _W - n
-	m := x[len(x)-1]
-	if len(z) > len(x) {
-		z[len(x)] = m >> ñ
-	}
-	for i := len(x) - 1; i >= 1; i-- {
-		y := x[i-1]
-		z[i] = m<<n | y>>ñ
-		m = y
-	}
-	z[0] = m << n
-	return z
-}
-
-
-func (z nat) shiftRightDeprecated(x nat, n uint) nat {
-	if len(x) == 0 {
-		return x
-	}
-
-	ñ := _W - n
-	m := x[0]
-	for i := 0; i < len(x)-1; i++ {
-		y := x[i+1]
-		z[i] = m>>n | y<<ñ
-		m = y
-	}
-	z[len(x)-1] = m >> n
-	return z
-}
-
-
 func (z nat) and(x, y nat) nat {
 	m := len(x)
 	n := len(y)
@@ -936,7 +882,7 @@ func (n nat) powersOfTwoDecompose() (q nat, k Word) {
 	x := trailingZeroBits(n[zeroWords])
 
 	q = q.make(len(n) - zeroWords)
-	q.shiftRightDeprecated(n[zeroWords:], uint(x))
+	shrVW(&q[0], &n[zeroWords], Word(x), len(q))
 	q = q.norm()
 
 	k = Word(_W*zeroWords + x)